To achieve it the way you currently do (not multiroom, but only able to view the same programme upstairs as what is showing downstairs) you would need a HDMI splitter (or an older Thomson box which supports two high definition outputs). Feed a single HDMI cable from the HD box into the splitter - then two HDMI cables out of the splitter and into your two high definition televisions.
If you can get hold of a Thomson box that supports both HDMI and component out - feed one TV from the HDMI output, the other from the component plus RCA stereo outs.
Both options obviously require fairly long cables running to the remote TV.
Multiroom, on top of the purchase price of the second HD box, will cost you an extra £19.50 - soon to move up to £20 - per month on top of what you currently pay. £9.75 for multiroom, £9.75 for a second HD Pack. And both boxes will need to be permanently connected to the same phoneline. But they will allow you to watch and record different channels upstairs to what the downstairs box is watching/recording.
If you go down the multiroom route you won't be able to watch items from the planner downstairs on your upstairs TV - or vice versa - unless you record the same programmes on both boxes. The boxes will run independently of each other.